Deep brain stimulation (DBS) refers to the delivery of electrical pulses into one or several specific sites within the brain of a patient to treat various neurological disorders. For example, DBS has been proposed as a clinical technique for treatment of chronic pain, essential tremor, Parkinson's disease (PD), dystonia, epilepsy, depression, obsessive-compulsive disorder, and other disorders.
A DBS procedure typically involves first obtaining preoperative images of the patient's brain (e.g., using computer tomography (CT) or magnetic resonance imaging (MRI)). The imaging process sometimes involves first affixing to the patient's skull fiducial markers that are discernable on the images produced by the imaging process. The fiducial markers assist in registering the preoperative images to the actual physical position of the patient in the operating room during the subsequent surgical procedure. Using the preoperative images, the neurosurgeon can select a target region within the brain, an entry point on the patient's skull, and a desired trajectory between the entry point and the target region. The entry point and trajectory are carefully selected to avoid intersecting or otherwise damaging critical brain structures.
In the operating room, the patient is immobilized and the patient's actual physical position is registered. The physician marks the entry point on the patient's skull and drills a burr hole at that location. A mechanism is provided to precisely control the path through the patient's brain to the desired location. Specifically, a positioning error on the order of a millimeter can have a significant negative effect on the efficacy of the DBS therapy. Stereotactic instrumentation and trajectory guide devices are commercially available products that facilitate the control of the trajectory and positioning of a lead during the surgical procedure.
A microdrive introducer can be used to insert a deep brain stimulation lead toward the selected region of the brain along the selected trajectory. The lead provides one or several conductive paths to deliver stimulation pulses to the selected region. The lead includes a very small diameter insulative lead body with one or several conductors (e.g., stranded wires) embedded in the insulative material. The lead also includes one or several electrodes at a distal end of the lead that are electrically coupled to respective conductors. The electrodes can be used to record signals within the brain and/or to deliver electrical stimulation pulses to brain tissue. Often, the electrical activity adjacent to one or several electrodes is analyzed to determine whether the recorded signals are consistent with the targeted region of the brain. If the recorded signals are not consistent with the targeted region, an adjustment to the lead's position can be made as appropriate.
After the correct location for the stimulation is established, trial stimulation may be provided. The purpose of trial stimulation is to determine whether the stimulation therapy may be effective for the patient's disorder and/or to determine appropriate stimulation parameters for the therapy. If suitable stimulation parameters are identified which are believed to permit treatment of the patient's disorder, an implantable pulse generator is implanted within the patient and the stimulation leads are tunneled to the implanted pulse generator.
One important stimulation parameter is the electrode combination used to apply stimulation to the neural tissue. Specifically, as electrodes are longitudinally distributed along a stimulation lead, selection of different electrodes to apply the electrical pulses changes the specific neural tissue being stimulation. Accordingly, the selection of different electrodes affects the quality of the therapy both in terms of benefits and adverse effects produced by the stimulation.
However, it is not always immediately evident during trial stimulation which electrode combination and other stimulation parameters are optimal. For example, refinement of stimulation parameters for depression may require a significant period of time before suitable stimulation parameters are identified. Specifically, the effectiveness of any single set of parameters is evaluated by monitoring the psychological state of the patient over a time and, hence, each revision of the parameters extends the evaluation process.